This invention relates generally to laser wax joinery, and more particularly to method and apparatus for automatic plural wax segment joinery in the production of patterns for investment casting.
In the manufacture of large castings, unitary wax models of the desired part are used to produce molds for use, in turn, in what is called a lost wax casting method. Such wax models are expensive to manufacture and usable only once.
One alternative to unitary wax model preparation, in the case of a large, circularly symmetric casting, is the segmentation of the wax model into preferably identical circumferential components that may be pieced together into a complete wax pattern. The joining of wax pattern segments for such large castings conventionally requires substantial manual labor and expense Such manual methods usually include preparation of each wax segment so as to conform the to-be-joined edges, and the use of hand tools, e.g. hot irons to weld abutting marginal edges of the wax segments and scrapers to restore the original surface characteristics and smoothness thereof.
Controlling the weld pool, that semi-liquid pool of wax within the range of the heat gun, in order to form a strong but smooth seam between the wax segments is problematic, especially with manual methods and tools. Achieving sufficient heat penetration for wax bonding, while avoiding loss of wax through dripping and vaporization, requires precise weld pool control. Attaining that measure of control, repeatably and cost effectively, requires, at least, semi-automation.
The feasibility of laser wax joinery has been determined experimentally, providing the basis for the method and apparatus of the present invention. Much has been learned about how precisely to control the wax weld pool by controlling laser beam power, beam diameter and welding rate, and by positioning or covering the weld pool to minimize wax shrinkage and dripping. As a result of this experimentation, and by the teachings of the present disclosure, it is possible accurately, repeatably and cost effectively to join wax segments under programmed control. It will be understood that wax segments, a used herein, refers to any two or more pieces of wax that may be joined, including the headers and risers that are needed to facilitate the injection molding process, as well as segment pairs that, when joined, represent the outline of the part to be cast.
Accordingly, it is an object of the present invention to provide method and apparatus for automatically joining wax pattern segments by the use of a laser.
Another object of the invention is to provide apparatus capable of forming smooth, finished seams requiring a minimum of manual rework.
A further object is to provide a method for controlling the wax weld pool produced by the laser welding of wax segments.
Yet another object of the invention is to provide a control system for automatically, repeatably joining plural wax segment pairs.
The preferred embodiment of the present invention includes a variable power, CO.sub.2 laser whose beam is directed robotically, under programmed control, along abutted marginal edges of the wax segment pairs to-be-joined. The segments of each pair are immobilized relative to one another in a fixture mounted on a tilt table Plural tilt tables are positioned circumferentially about a process robot's laser-equipped arm. According to the preferred method of the invention, the wax weld pool precisely is controlled via control of the laser beam's energy density (power, diameter and scan rate), while maintaining, substantially horizontal, that portion of the abutted wax segment pair to which the laser beam is directed.
In a modification to the preferred method, the weld pool is controlled by the preparatory covering of the seam to be welded with a flow-resistant, conformable, but substantially laser beam-transmissive polyethylene material. In a modification to the covering step, a flow-resistant, conformable, thermally conductive, metal foil material is used, the material preferably being painted black on one side to reduce reflection of laser energy and having an adhesive preparation on the other. In another modification to the preferred method, the weld pool is controlled by the application of air pressure to restore the molten wax to its original position. In yet another modification, the laser beam's energy density is reduced, and the beam repeatedly is scanned across a portion of the seam to be welded, thus avoiding wax dripping or filler vaporization. In still another modification, the wax segments' to-be-joined edges are prepared to be joggle-lap, rather than planar abuttment, joined further to increase the strength of the resulting weld joint.
By the method and apparatus of the present invention, strong, smooth, weld joints repeatably may be made in the joining of plural wax segments, and wax patterns for investment casting may be produced in a fully automated process that significantly reduces costs.
These and other advantages and features of the invention will become more fully apparent when the detailed description below is read with reference to the accompanying drawings.